About Air Temperature Data

Introduction

Air temperature: The temperature of air is a measure of the average thermal energy of the molecules in the air - the higher the temperature, the higher the energy of the molecules. Historically, the most common instrument used has been a liquid-in-glass thermometer mounted in a Stevenson screen. Modern automatic weather stations generally use an electronic sensor. The daily maximum (highest) and minimum (lowest) temperature are generally recorded once each day at 9 am local time, while instantaneous measurements of air temperature may be made at various times throughout the day.

The daily (24 hour) maximum and minimum air temperatures are nominally recorded at 9 am local time. The highest temperature for the 24 hours leading up to the observation is recorded as the maximum temperature for the previous day, while the lowest temperature over the previous 24 hours is recorded as the minimum temperature for the day on which the observation was made. If, for some reason, an observation is unable to be made, the next observation is recorded as an accumulation, since there has been more than one day over which the maximum and minimum temperature may have occurred.

Temperature data prior to 1910 should be used with extreme caution as many stations, prior to that date, were exposed in non-standard shelters, some of which give readings which are several degrees warmer or cooler than those measured according to post-1910 standards.

Daily maximum temperatures usually occur in the afternoon and daily minimum temperatures overnight or near dawn. Occasionally, however, the lowest temperature in the 24 hours to prior to 9 am can occur around 9 am the previous day if the night was particularly warm.

There are historical maximum and minimum temperature data for around 1800 stations, with about 800 stations currently making observations. Of these, approximately 500 are automatic weather stations (AWS). Some stations which were operational before the mid 1900s may have more years of monthly temperature data available than corresponding daily temperature observations. During those years (before computer data storage), one practice to asist in managing the growing number of observations was to enter into the central climate data archive only a summary of the month (e.g. highest, lowest, and mean maximum temperature) rather than all the observations for each day.

General information about historical observations

Data Quality

Climate data pass through a number of stages in quality control which occurs over a period of time. Data are only included in this product if no errors have been detected at the time. Data which have not yet completed the routine quality control process are marked accordingly.

Gaps and missing data

Very few stations have a complete unbroken record of climate information. A station may have been closed, reopened, upgraded to a full weather station or downgraded to a rainfall only station during its existence causing breaks in the record for some or all elements. Some gaps may be for one element due to a damaged instrument, others may be for all elements due to the absence or illness of an observer, or perhaps the failure of an automatic weather station.

Instruments and observational practices

Historically, if a station moved a relatively short distance (within about 1 to 2 km) it may, but not always, have continued to use the same station number. Changes may have occurred in instrumentation and/or observing practices over the period included in a dataset, which may have an effect on the long-term record. In recent years many stations have had observers replaced by Automatic Weather Stations, either completely or at certain times of the day.

Latitudes and longitudes

The accuracy of the geographic coordinates of a station has generally improved since the Bureau started taking observations. In the early days the position of a station was usually estimated from a map. In recent years, as an open station was inspected the location was checked (and if necessary corrected) using GPS (Global Positioning System).

Time

For a part of the year some Australian States adopt Daylight Savings Time (DST). The changeover dates vary from State to State and year to year. More information can be found on the following page:
http://www.bom.gov.au/climate/averages/tables/daysavtm.shtml
Observing practices, such as whether an observation was made according to local standard time or local clock time (incorporating DST if applicable), have undergone some changes since observations began. More information is available at:
http://www.bom.gov.au/climate/averages/tables/dst_obs.shtml

Copyright

The copyright for any data is held in the Commonwealth of Australia and the purchaser shall give acknowledgement of the source in reference to the data. Apart from dealings under the copyright Act, 1968, the purchaser shall not reproduce, modify or supply (by sale or otherwise) these data without written permission. For further information please refer to the Bureau of Meteorology's Copyright Notice.

Liability

While every effort is made to supply the best data available this may not be possible in all cases. We do not give any warranty, nor accept any liability in relation to the information given, except that liability (if any), that is required by law.

Page updated: September 13, 2011